Method of transmitting reference signal and transmitter using the same

ABSTRACT

A method and apparatus of transmitting a reference signal in a wireless communication system is provided. A reference signal sequence is generated by using a pseudo-random sequence. A portion or entirety of the reference signal sequence is mapped to at least one resource block and is transmitted. The pseudo-random sequence is generated by a gold sequence generator which is initialized with initial values obtained by using cell identifier. The reference signal provides low PAPR and high cross correlation characteristic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending application Ser. No.12/405,686 filed on Mar. 17, 2009, which claims the benefit of priorityof U.S. Provisional Application No. 61/036,998 filed on Mar. 17, 2008,U.S. Provisional Application No. 61/048,227 filed on Apr. 28, 2008, U.S.Provisional Application No. 61/049,777 filed on May 2, 2008, and KoreanPatent Application No. 10-2009-0021828 filed on Mar. 13, 2009. All theseapplications are incorporated by reference in their entirety herein.

BACKGROUND

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to generation and application of a sequence for areference signal in a wireless communication system.

2. Related Art

Wireless communication systems are widely spread all over the world toprovide various types of communication services such as voice or data.In general, the wireless communication system is a multiple accesssystem capable of supporting communication with multiple users bysharing available system resources (e.g., bandwidth, transmit power,etc.). Examples of the multiple access system include a code divisionmultiple access (CDMA) system, a frequency division multiple access(FDMA) system, a time division multiple access (TDMA) system, anorthogonal frequency division multiple access (OFDMA) system, a singlecarrier frequency division multiple access (SC-FDMA) system, etc.

In the wireless communication system, a sequence is generally used invarious usages such as a reference signal, a scrambling code, etc. Thesequence used in the wireless communication system generally satisfiesthe following properties.

(1) A good correlation property for providing high detectionperformance.

(2) A low peak-to-average power ratio (PAPR) for increasing efficiencyof a power amplifier.

(3) Generation of a large number of sequences to transmit a large amountof information or to facilitate cell planning.

Although a constant amplitude and zero auto correlation (CAZAC) sequencehaving a good PAPR property has been proposed, the number of availablesequences is limited. Therefore, many wireless communication systems usea sequence generated in a pseudo-random manner. A pseudo-random sequencehas an advantage in that a large number of sequences are available, buthas a problem of a high PAPR in a specific pattern.

Various binary or non-binary pseudo-random sequences have been used inthe wireless communication system. The pseudo-random sequences can beeasily generated using an m-stage linear feedback shift register (LFSR),and have a significantly excellent random property. An m-sequence isused as a scrambling code in a wideband CDMA (WCDMA) system since astructure of the m-sequence is simpler than the non-binary pseudo-randomsequence.

A gold sequence is a pseudo-random sequence generated by using twodifferent binary m-sequences. The gold sequence can be easilyimplemented by two m-stage LFSRs. The gold sequence has an advantage inthat different pseudo-random sequences can be generated in accordancewith a period while varying an initial state of each m-stage LFSR.

Accordingly, there is a need for a method capable of generating asequence with improved PAPR and correlation properties.

SUMMARY

The present invention provides a method and apparatus for transmitting areference signal in a wireless communication system. In addition, areceiver for receiving the transmitted reference signal is alsoprovided.

The present invention also provides a method and apparatus fortransmitting a sequence in a wireless communication system. In addition,a receiver for receiving the transmitted sequence is also provided.

In an aspect, a method of transmitting a reference signal in a wirelesscommunication system is provided. The method includes generating areference signal sequence, mapping a portion or entirety of thereference signal sequence to at least one RB, and transmitting areference signal in the at least one RB. The reference signal sequenceis defined by

${{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2m} + 1} \right)}}} \right)}}},{m = 0},1,\ldots \mspace{14mu},{{2N_{RB}^{\max,{DL}}} - 1}$

where n_(s) is a slot number within a radio frame, l is an orthogonalfrequency division multiplexing (OFDM) symbol number within a slot andN_(RB) ^(max,DL) is a maximum number of resource blocks (RBs). Apseudo-random sequence c(i) is generated by a gold sequence generatorwhich is initialized with initial values obtained by using (2N_(ID)^(cell)+1), where N_(ID) ^(cell) is a cell identifier.

The pseudo-random sequence c(i) may be defined by

c(i)=(x(i+Nc)+y(i+Nc))mod 2

x(i+31)=(x(i+3)+x(i))mod 2

y(i+31)=(y(i+3)+y(i+2)+y(i+1)+y(i))mod 2

where x(i) and y(i) are m-sequences and Nc is a constant. The m-sequencex(i) may be initialized with x(0)=1, x(i)=0, i=1, 2, . . . , 30, and them-sequence (y) may be initialized with the initial values. The Nc may bea value in range from 1500 to 1800.

The initial values may vary as the OFDM symbol number varies. Theinitial values may be obtained by using 1 (2_(ID) ^(cell)+1). The sizeof the initial values may be 31 bits.

One RB may comprise 12 subcarriers in frequency domain. Two modulationsymbols of the reference signal sequence may be mapped to twosubcarriers in one RB.

The reference signal may be a cell common reference signal or a userequipment (UE) specific reference signal.

In another aspect, a transmitter includes a reference signal generatorto generate a reference signal, and a transmit circuitry to transmit thereference signal. The reference signal generator generates the referencesignal by

generating a reference signal sequence which is defined by

${{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)}}},{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}$

where n_(s) is a slot number within a radio frame, 1 is an OFDM symbolnumber within a slot and N_(RB) ^(max,DL) is a maximum number of RBs. Apseudo-random sequence c(i) is generated by a gold sequence generatorwhich is initialized with initial values obtained by using (2 N_(ID)^(cell)+1), where N_(ID) ^(cell) is a cell identifier. The referencesignal generator maps a portion or entirety of the reference signalsequence to at least one RB.

In still another aspect, a receiver includes a receive circuitry toreceive a reference signal and a receive signal, a channel estimator toestimate a channel by using the reference signal, and a data processorto process the receive signal by using the channel. The reference signalis generated based on a reference signal sequence which is defined by

${{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)}}},{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}$

where n_(s) is a slot number within a radio frame, 1 is an OFDM symbolnumber within a slot and N_(RB) ^(max,DL) is a maximum number of RBs. Apseudo-random sequence c(i) is generated by a gold sequence generatorwhich is initialized with initial values obtained by using (2 N_(ID)^(cell)+1), where N_(ID) ^(cell) is a cell identifier.

A proposed sequence provides low peak-to-average power ratio (PAPR) andhigh cross-correlation properties. Therefore, transmit power can beeffectively provided in a transmitter, and signal detection performancecan be improved in a receiver. The proposed sequence can be used for areference signal requiring high reliability and also can be used forother scrambling codes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows a structure of a radio frame in the 3GPP LTE.

FIG. 3 shows an example of a resource grid for one downlink slot.

FIG. 4 shows an exemplary structure of a downlink subframe.

FIG. 5 shows an exemplary structure of a downlink common referencesignal when a BS uses one antenna.

FIG. 6 shows an exemplary structure of a downlink common referencesignal when a BS uses two antennas.

FIG. 7 shows an exemplary structure of a downlink common referencesignal when a BS uses four antennas.

FIG. 8 shows an example of a gold sequence generator.

FIG. 9 shows setting of the initial values of the second LFSR.

FIG. 10 is a graph for comparing sizes of a reference signal and anydata when initial values of the second LFSR are all set to ‘0’.

FIG. 11 shows a problem caused by initial values of a gold sequence in amulti-cell environment.

FIG. 12 shows an example where bit sequences, which are cyclicallymapped in QPSK modulation, are set to initial values.

FIG. 13 shows an example where the initial values of the first LFSR areset to 1's complements of the initial values of the second LFSR.

FIG. 14 shows that an offset of an available sequence varies accordingto a cell ID.

FIG. 15 shows that a basic sequence in use is cyclic shifted accordingto a cell ID.

FIG. 16 shows that the start point of the used sequence is changedaccording to the subframe number and/or the OFDM symbol number.

FIG. 17 shows setting of initial values of a gold sequence generator.

FIG. 18 is a flowchart showing a method of transmitting a referencesignal according to an embodiment of the present invention.

FIG. 19 is a block diagram showing a transmitter and a receiverimplementing for a method of transmitting and receiving a referencesignal.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technique described below can be used in various wireless accesstechnologies such as code division multiple access (CDMA), frequencydivision multiple access (FDMA), time division multiple access (TDMA),orthogonal frequency division multiple access (OFDMA), single carrierfrequency division multiple access (SC-FDMA), etc. The CDMA may beimplemented with a radio technology such as Universal Terrestrial RadioAccess (UTRA) or CDMA2000. The TDMA may be implemented with a radiotechnology such as Global System for Mobile communications (GSM)/GeneralPacket Radio Service (GPRS)/Enhanced Data Rates for GSM Evolution(EDGE). The OFDMA may be implemented with a radio technology such asinstitute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi),IEEE 802.16 (WiMAX), IEEE 802-20, evolved-UTRA (E-UTRA) etc. The UTRA isa part of a universal mobile telecommunication system (UMTS). 3rdgeneration partnership project (3GPP) long term evolution (LTE) is apart of an evolved-UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE employsthe OFDMA in downlink and employs the SC-FDMA in uplink. LTE-advance(LTE-A) is an evolution of the 3GPP LTE.

For clarity, the following description will focus on the 3GPP LTE/LTE-A.However, technical features of the present invention are not limitedthereto.

FIG. 1 shows a wireless communication system.

Referring to FIG. 1, a wireless communication system 10 includes atleast one base station (BS) 11. The BSs 11 provide communicationservices to specific geographical regions (generally referred to ascells) 15 a, 15 b, and 15 c. The cell can be divided into a plurality ofregions (referred to as sectors). A user equipment (UE) 12 may be fixedor mobile, and may be referred to as another terminology, such as amobile station (MS), a user terminal (UT), a subscriber station (SS), awireless device, a personal digital assistant (PDA), a wireless modem, ahandheld device, etc. The BS 11 is generally a fixed station thatcommunicates with the UE 12 and may be referred to as anotherterminology, such as an evolved node-B (eNB), a base transceiver system(BTS), an access point, etc.

Hereinafter, a downlink is a communication link from the BS to the UE,and an uplink is a communication link from the UE to the BS. In thedownlink, a transmitter may be a part of the BS, and a receiver may be apart of the UE. In the uplink, the transmitter may be a part of the UE,and the receiver may be a part of the BS.

FIG. 2 shows a structure of a radio frame in the 3GPP LTE.

Referring to FIG. 2, the radio frame includes 10 subframes. One subframeincludes two slots. A time for transmitting one subframe is defined as atransmission time interval (TTI). For example, one subframe may have alength of 1 millisecond (ms), and one slot may have a length of 0.5 MS.

One slot includes a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain. Since the 3GPP LTE usesthe OFDMA in the downlink, the OFDM symbol is for representing onesymbol period. According to a system, the OFDM symbol can also bereferred to as an SC-FDMA symbol or a symbol period. A resource block(RB) is a resource allocation unit, and includes a plurality ofcontiguous subcarriers in one slot.

The structure of the radio frame is shown for exemplary purposes only.Thus, the number of subframes included in the radio frame or the numberof slots included in the subframe or the number of OFDM symbols includedin the slot may be modified in various manners.

FIG. 3 shows an example of a resource grid for one downlink slot.

Referring to FIG. 3, the downlink slot includes a plurality of OFDMsymbols in a time domain. It is described herein that one downlink slotincludes 7 OFDM symbols, and one resource block (RB) includes 12subcarriers in a frequency domain as an example. However, the presentinvention is not limited thereto.

Each element on the resource grid is referred to as a resource element.One RB includes 12×7 resource elements. The number N^(DL) of RBsincluded in the downlink slot depends on a downlink transmit bandwidth.

FIG. 4 shows an exemplary structure of a downlink subframe.

Referring to FIG. 4, the subframe includes two slots. A maximum of threeOFDM symbols located in a front portion of a 1^(st) slot within thesubframe correspond to a control region to be assigned with controlchannels. The remaining OFDM symbols correspond to a data region to beassigned with a physical downlink shared chancel (PDCCH).

Examples of downlink control channels used in the 3GPP LTE includes aphysical control format indicator channel (PCFICH), a physical downlinkcontrol channel (PDCCH), a physical hybrid ARQ indicator channel(PHICH), etc. The PCFICH is transmitted at a first OFDM symbol of asubframe and carries information regarding the number of OFDM symbolsused for transmission of control channels within the subframe. Controlinformation transmitted through the PDCCH is referred to as downlinkcontrol information (DCI). The DCI includes uplink or downlinkscheduling information or includes an uplink transmit power controlcommand for arbitrary UE groups.

Now, a reference signal will be described.

When data is transmitted in a wireless communication system, thetransmitted data may be distorted on a radio channel. In order for areceiver to restore the distorted data into original data, a channelstate needs to be known so that distortion of a received signal iscompensated for according to the channel state. To know the channelstate, a signal known in advance to both a transmitter and the receiveris used. Such a signal is referred to as a reference signal or a pilot.Since the reference signal is an important signal to know the channelstate, the transmitter transmits the reference signal with greatertransmit power than other signals. In addition, to distinguish thereference signal transmitted between cells in a multi-cell environment,the reference signal has to have good peak-to-average power ratio (PAPR)and correlation properties.

The reference signal can be classified into a cell common referencesignal and a UE specific reference signal. The cell common referencesignal is a reference signal used by all UEs within a cell. The UEspecific reference signal is a reference signal used by a UE within thecell or used by a UE group.

FIG. 5 shows an exemplary structure of a downlink common referencesignal when a BS uses one antenna. FIG. 6 shows an exemplary structureof a downlink common reference signal when a BS uses two antennas. FIG.7 shows an exemplary structure of a downlink common reference signalwhen a BS uses four antennas. This may be found in section 6.10.1 of3GPP TS 36.211 V8.0.0 (2007-09) “Evolved Universal Terrestrial RadioAccess (E-UTRA); Physical channels and modulation (Release 8)”. Rpdenotes a reference signal for a p-th antenna (herein, pε{0, 1, 2, 3}).R0 to R3 do not overlap with one another. In one OFDM symbol, each Rp ispositioned with a spacing of 6 subcarriers. Therefore, if one RBincludes 12 subcarriers, a sequence having a length of 2 sequences (ortwo modulation symbols) is required for one RB. Within a subframe, thenumber of R0s is equal to the number of R1s, and the number of R2s isequal to the number of R3s. Within the subframe, the number of R2s andR3s is less than the number of R0s and R1s. Rp is not used intransmission through antennas except for the p^(th) antenna. This is toavoid interference between antennas.

Now, generation of a sequence for a reference signal will be described.

A reference signal generated by using a gold sequence generator isconsidered. A gold sequence can be implemented with two 31-stage linearfeedback shift registers (LFSRs). It is assumed that a first LFSR‘x(30)x(29)x(28) . . . x(2)x(1)x(0)’ of the two LFSRS is initializedwith ‘0000000000000000000000000000001’. In addition, initial values of asecond LFSR are determined by a cell identifier (ID), a subframe number,and an OFDM symbol number. The cell ID denotes a cell specific ID. Thesubframe number denotes an index of a subframe within a radio frame. TheOFDM symbol number denotes an index of an OFDM symbol within a subframe(or slot).

FIG. 8 shows an example of a gold sequence generator. A sequencegeneration polynomial D³¹+D³+1 is used for a first m-sequence x(i), anda sequence generation polynomial D³¹+D³+D²+D+1 is used for a secondm-sequence y(i). These two m-sequences are used to generate apseudo-random sequence c(i). The pseudo-random sequence c(i) isgenerated by a generation polynomial of Equation 1 as shown:

c(i)=(x(i)+y(i))mod 2

x(i+31)=(x(i+3)+x(i))mod 2

y(i+31)=(y(i+3)+y(i+2)+y(i+1)+y(i))mod 2  [Equation 1]

where i=0, 1, . . . , M_(max)−1, and M_(max) is a length of a binarypseudo-random sequence generated using a gold sequence. In thepseudo-random sequence having a length of Mmax, only a portion of thesequence may be used. If M is a length of a sequence using only aportion of the pseudo-random sequence having a length of M_(max), thenM≦M_(max). M may vary depending on the number of RBs used for datatransmission. The number of available RBs varies according to anavailable frequency band in a 3GPP LTE system, and thus the value M mayalso vary according to the number of allocated RBs.

In case of the first LFSR, the initial values are fixed to‘0000000000000000000000000000001’ as described above. The initial valuesof the second LFSR are determined by the cell ID, the subframe number,and the OFDM symbol number.

FIG. 9 shows setting of the initial values of the second LFSR. Among the31 bits of the initial values, 17 bits from a least significant bit(LSB) are initialized with a 9-bit cell ID, a 4-bit subframe number, anda 4-bit OFDM symbol number. The 3GPP LTE supports 504 unique cell IDs,and thus the cell ID ranges from 0 to 503. One radio frame includes 10subframes, and thus the subframe number ranges from 0 to 9. One subframecan include up to 14 OFDM symbols, and thus the OFDM symbol numberranges from 0 to 13. The remaining 14 bits from a most significant bit(MSB) are initialized with ‘0’. The initial values of the second LFSRcan be expressed by the following table.

TABLE 1 x(30) x(29) x(28) x(27) x(26) . . . x(3)x(2)x(1)x(0) Set to zeroOFDM symbol number Subframe Number Cell ID 14 bit 4 bit 4 bit 9 bit

In the above table 1, a range and/or bit number for cell ID, OFDM symbolnumber and subframe number are exemplary purposed only and are notlimited thereto. For example, the subframe number may be represented asa slot number. Since a radio frame includes 20 slots, the slot numbermay be in a range of 0-19.

After determining the initial values of the first LFSR and the initialvalues of the second LFSR, a portion or entirety of the pseudo-randomsequence generated by the gold sequence generator is used as a referencesignal. The generated sequence is modulated into a modulation symbolthrough quadrature phase shift keying (QPSK)modulation, and then ismapped to each resource element. In this case, only a portion of apre-generated, specific-sized gold sequence can be used. For example, asshown in FIG. 5, among the 12 subcarriers constituting one RB, thereference signal can be mapped to two subcarriers with a spacing of 6subcarriers.

However, when the pseudo-random sequence is generated as describedabove, ‘0’ and ‘1 may not be included with a similar ratio in thegenerated binary pseudo-random sequence, and thus the number of ‘0’s maybe greater than the number of ‘1's or the number of ‘1’s may be greaterthan the number of ‘0’s. In this case, even if QPSK modulation isperformed, a direct current (DC) component exists due to the biasedpseudo-random sequence. As a result, the PAPR property may deterioratethrough a process of inverse fast Fourier transform (IFFT). As arepresentative example, if the cell ID, the subframe number, and theOFDM symbol number are all ‘0’, the 31 bits of the initial values of thesecond LFSR are all set to ‘0’.

FIG. 10 is a graph for comparing sizes of a reference signal and anydata when initial values of the second LFSR are all set to ‘0’. When 31bits of initial values of the gold sequence generator are allinitialized with ‘0’, a reference signal having a significantly largesize than any other data is generated at some time samples, whichimplies degradation of the PAPR property.

FIG. 11 shows a problem caused by initial values of a gold sequence in amulti-cell environment. In the multi-cell environment, each cell has aunique cell ID. However, since only 9 bits out of 31 bits of the initialvalues are different, a pseudo-random sequence may be generated to bealmost identical for each cell if the remaining 22 bits are identical.In particular, if the cell ID is contiguous in each cell, up to 30 bitsoutput the 31 bits of the initial values may overlap. Therefore, thecorrelation property may deteriorate when the generated pseudo-randomsequence is used as a reference signal.

Hereinafter, generation of a sequence and application of the generatedsequence for solving the aforementioned problem will be described.

First, a method of generating a sequence by changing a most significantbit (MSB) of initial values of a gold sequence will be described.

When a pseudo-random sequence is generated using the gold sequence, 14bits from the MSB are changed to a suitable value to equalize a ratio of‘0’ and ‘1’ included in the initial values of the second LFSR. Since acell ID, a subframe number, and an OFDM symbol number may be all set to‘0’ in some cases, the remaining 14 bits are changed to a suitable valueto define a gold sequence having a good PAPR property. In case of usingthe gold sequence, the generated pseudo-random sequence is determinedaccording to the initial values. Thus, it is important to set theinitial values to generate a sequence having a good PAPR.

In one embodiment, 14 bits from the MSB may be all set to ‘1’. By doingso, the initial values of the gold sequence can be prevented from beingall set to ‘0’. Thus, deterioration of the PAPR property can be avoided.Table 2 shows a PAPR when the 14 bits from the MSB are all set to ‘0’.Table 3 shows a PAPR when the 14 bits from the MSB are all set to ‘1’.In Table 2 and Table 3, a peak value is shown with respect to the numberof RBs (i.e., 6, 12, 25, 50, and 100) when a reference signal in use isa basic sequence generated by differently setting 17 bits from the LSBof the gold sequence generator according to a cell ID, a subframenumber, and an OFDM symbol number.

TABLE 2 # of RB MSB LSB Peak value PAPR 6 0000000000000010000110000100110 1.06 1.89 12 00000000000000 10000011010110011 1.332.36 25 00000000000000 00000000000000000 2.18 3.71 50 0000000000000000000000000000000 5.16 8.81 100 00000000000000 00000000000000000 10.6018.10

TABLE 3 # of RB MSB LSB Peak value PAPR 6 1111111111111110110000011101000 0.93 1.66 12 11111111111111 10110101111110010 1.282.28 25 11111111111111 10110110110110110 1.53 2.61 50 1111111111111110010100100100100 1.87 3.19 100 11111111111111 00000000000000010 2.494.25

As shown in Table 2 and Table 3, when the 14 bits from the MSB are allset to ‘0’, the PAPR property is superior to a case where the 14 bitsfrom the MSB are all set to ‘1’.

In another embodiment, the 14 bits from the MSB may be set to a bitsequence that can be cyclically mapped on a QPSK constellation. Sequencevalues initially output from the gold sequence generator are the same asthe initial values. Thus, when the initial values are uniformly arrangedon 4 symbol positions on the QPSK constellation, modulation symbols ofthe generated pseudo-random sequence can be prevented from beingconcentrated on a specific QPSK modulation symbol.

FIG. 12 shows an example where bit sequences, which are cyclicallymapped in QPSK modulation, are set to initial values. Assume that bitsequences ‘00’, ‘01’, ‘11’, and ‘10’ on the QPSK constellationrespectively correspond to modulation symbols {circle around (1)},{circle around (2)}, {circle around (3)}, and {circle around (4)}. Thebit sequences are set so that 4 modulation symbols uniformly appear in14 bits from an MSB. First, a first bit sequence ‘0011110000111’ isdefined so that the modulation symbols appear in an order of {circlearound (1)}, {circle around (2)}, {circle around (3)}, {circle around(4)}, {circle around (1)}, {circle around (2)}, {circle around (3)}. Inpractice, an output of the gold sequence generator starts from an LSB.Thus, a second bit sequence ‘11100001111000’ is defined by inversion ofthe first bit sequence. 17 bits from the LSB are set to values givenaccording to a cell ID, a subframe number, and an OFDM symbol number,and one QPSK modulation symbol consists of 2 bits. Thus, a thirdsequence ‘11000011110001’ is generated by cyclic-shifting the second bitsequence leftward by 1 bit. Among the 14 bits from the MSB, a bitnearest to the 17 bits from the LSB are randomly set, and bitssubsequent to the nearest bit (i.e., a 19^(th) bit from the LSB) aremapped to one modulation symbol in a unit of 2 bits. Consequently, if amodulation symbol is output starting from the LSB, in case of the thirdsequence, the modulation symbol is output in an order of {circle around(1)}, {circle around (2)}, {circle around (3)}, {circle around (4)},{circle around (1)}, {circle around (2)}.

Table 4 shows the PAPR property according to the number of RBs when the14 bits of the MSB are set to ‘11000011110001’.

TABLE 4 # of RB MSB LSB Peak value PAPR 6 1100001111000110110000010110111 0.96 1.71 12 11000011110001 01100100010101001 1.312.33 25 11000011110001 01110101011011011 1.42 2.42 50 1100001111000101111001101100100 1.70 2.90 100 11000011110001 00110000011001010 2.063.52

As shown in Table 4, the PAPR property is improved when the 14 bits fromthe MSB are set to proposed values.

In another embodiment, various combinations of the 14 bits from the MSBare proposed to improve the PAPR property. The 14 bits from the MSB canbe changed from ‘00000000000000’ to ‘11111111111111’ in order to find avalue having an optimal PAPR property for all possible cases, whichresults in significantly large complexity. It is assumed herein that thenumber of RBs is 6, 12, 25, 50, or 100, and a reference signal in use isa sequence having a length corresponding to the number of RBs. For eachnumber of RBs, 17 bits from the LSB are differently set according to acell ID, a subframe number, and an OFDM symbol number. The referencesignal is subjected to an IFFT operation for OFDM modulation, and if apeak value of an OFDM symbol that is a time-domain signal exceeds aspecific threshold, the OFDM symbol is removed from candidates.

Table 5 shows 14 bits from the MSB having a best PAPR property for eachnumber of RBs (i.e., 6, 12, 25, 50, and 100).

TABLE 5 # of RB MSB LSB Peak value PAPR 6 0001000111000100000001110000010 0.89 1.58 12 11001100100000 01000110110110101 1.101.96 25 01011111100110 00000011011100011 1.28 2.19 50 0110011001010100010110100100101 1.42 2.42 100 00100001000101 01100100011010000 1.442.46

When an optimal value shown in Table 5 is used in the 14 bits from theMSB according to each number of RBs, increase of a PAPR caused by biascan be prevented.

Table 6 shows a peak value and a PAPR when the 14 bits from the MSB(i.e., ‘00010001110001’) of Table 5 are used for each number of RBs. Itshows that, when an optimal value for a specific number of RBs is usedfor a different number of RBs, the optimal value may not be optimal.

TABLE 6 # of RB MSB LSB Peak value PAPR 6 0001000111000100000001110000010 0.89 1.58 12 00010001110001 01000011010101011 1.502.67 25 00010001110001 01000000010110001 1.51 2.58 50 0001000111000110110011000011001 1.68 2.86 100 00010001110001 00010100111100111 1.752.99

To be selected as the optimal value, it is important to have uniformPAPR characteristics over multiple RBs. When the optimal value is set toa value which has the smallest sum of the peak values for each RB amongvalues not to exceeds a specific threshold, the 14 bits from the MSB,‘00111101101100’, is selected as the optimal value. Table 7 shows a peakvalue and a PAPR when the 14 bits from the MSB, ‘00111101101100’ ofTable 7 are used for each number of RBs.

TABLE 7 # of RB MSB LSB Peak value PAPR 6 0011110110110001100011010011001 0.89 1.59 12 00111101101100 00001000101010101 1.142.03 25 00111101101100 00000111111001000 1.40 2.40 50 0011110110110001001001010101000 1.55 2.65 100 00111101101100 10010011111001000 1.562.66

The PAPR property deteriorates in comparison with the result of Table 5,which can be regarded as an optimum, but the PAPR property shows abetter result than the result of Table 6 in which the 14 bits from theMSB (i.e., ‘00010001110001’) are used. Thus, the peak value and the PAPRproperty are uniform as a whole. Accordingly, complexity can be lowerthan a case of using the 14 bits from the MSB differently according tothe number of RBs, and has an advantage in that a memory size isreduced.

A method of improving the PAPR property by setting the initial values ofthe second LFSR of the gold sequence generator has been described above.Hereinafter, a method of improving the PAPR property of a sequence bysetting the initial values of the first LFSR will be described.

In one embodiment, the initial value of the first LFSR can be defined tospecific values. For example, bit sequences to which modulation symbolscan be uniformly mapped on a QPSK constellation are set to the initialvalues. If bit sequences ‘00’, ‘01’, ‘11’, and ‘10’ are reversely sorted(this is because an LSB is first output in the gold sequence) andmapping is repeated only up to 31 bits, a resultant value is‘111000011110000111100001111000’. Table 8 shows a peak value and a PAPRaccording to the number of REs and initial values of the first LFSR whenthe initial values of the first LFSR is‘1111000011110000111100001111000’. The PAPR is significantly decreasedin comparison with the result of Table 2.

TABLE 8 # of RB MSB LSB Peak value PAPR 6 0000000000000000000011110010000 0.95 1.69 12 00000000000000 01110010110011100 1.162.07 25 00000000000000 01110001110001110 1.77 3.02 50 0000000000000001100010011001001 1.86 3.18 100 00000000000000 10010111111011000 1.742.97

In another embodiment, the initial values of the first LFSR can be setto l′s complements of the initial values of the second LFSR. FIG. 13shows an example where the initial values of the first LFSR are set tol′s complements of the initial values of the second LFSR. Even if theinitial values of the second LFSR of the gold sequence generator are setto ‘0’, the initial values of the first LFSR are all set to 1'scomplements of the initial values of the second LFSR. Accordingly, asequence having a more random property can be generated, and thusdeterioration of the PAPR property can be prevented. Table 9 shows aresult when the initial values of the first LFSR are set to 1'scomplements of the initial values of the second LFSR.

TABLE 9 # of RB MSB LSB Peak value PAPR 6 0000000000000000000000000001000 0.97 1.72 12 00000000000000 00010001010101010 1.272.26 25 00000000000000 00010101010101010 2.22 3.78 50 0000000000000001110001110001110 2.98 5.08 100 00000000000000 00010100100000110 3.916.68

Meanwhile, to distinguish a reference signal between cells or betweenUEs, the reference signal has to have a good correlation property. Asdescribed with reference to FIG. 11, in the initial values of the goldsequence generator, if only a cell ID differs and other values (i.e., asubframe number and an OFDM symbol number) are identical, a generatedpseudo-random sequence may equally overlap in some periods. This occursbecause only values of 9 bits of the initial values are different among31 bits of the initial values. This problem may be solved by consideringa fact that only a portion of a sequence generated as a reference signalis used. This is because, even if a pseudo-random sequence having alength of M_(max) (this sequence is referred to as a basic sequence) isgenerated, a sequence having a length of M (this sequence is referred toas a used sequence) is used according to the number of RBs. Thus, if theused sequence is selected at different offsets from basic sequencesgenerated according to the cell ID, it is possible to solve a problem inthat the sequence overlaps in a portion of period due to almostidentical initial values.

Now, a method of setting an offset of a sequence according to a cell IDwill be described.

It is assumed that a basic sequence having a length of Mmax, i.e., abasic sequence c(i) (i=0, 1, . . . , M_(max)−1), is generated by thegold sequence generator, and then a used sequence having a length of Mis used. In this case, M≦M_(max). An offset of the used sequence, i.e.,a start point of the used sequence, is set differently according to thecell ID.

FIG. 14 shows that an offset of an available sequence varies accordingto a cell ID. Herein, an offset is placed with a spacing of N in a basicsequence having a length of M_(max) according to the cell ID, and a usedsequence having a length of M is selected. The used sequence is cyclicshifted when exceeding a range of the basic sequence. From the basicsequence c(i) (i=0, 1, . . . , M_(max)−1), a used sequence cu(i) (i=0,1, . . . , M−1) can be expressed as shown:

cu(i)=c((i+N·N _(ID) ^(cell))mod(M _(max)−1))  [Equation 2]

where ‘mod’ is a modulo operation, N is an offset interval, and N_(IC)^(cell) is a cell ID. Although the same offset is defined for each cellID herein, this is for exemplary purposes only, and thus the offset maybe defined differently for each cell ID.

By varying a start point of the used sequence according to the cell ID,the used sequence may vary even if initial values are similar.Therefore, a random property can be guaranteed, and the PAPR propertycan be prevented from deterioration.

Equation 2 above can be expressed in a format of a reference signal forthe 3GPP LTE system in which resources are allocated in an RB unit,which is shown in the following equation.

$\begin{matrix}{\begin{matrix}{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} +}} \\{{{j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)},}}\end{matrix}{{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}}{a_{k,l}^{(p)} = {r_{l,n_{s}}\left( m^{\prime} \right)}}{{m = 0},1,\ldots \mspace{14mu},{{2\; \cdot N_{RB}^{DL}} - 1}}{m^{\prime} = {\left( {m + {N_{Interval}^{RS}N_{ID}^{cell}}} \right){{mod}\left( {{2 \cdot N_{RB}^{\max,{DL}}} - 1} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Herein, n_(s) is a slot number within a radio frame, 1 is an OFDM symbolnumber within a slot, r_(l,ns) is a reference signal sequence, N_(RB)^(max,DL) is a maximum number of RBs, m is an index of the referencesignal sequence, m′ is an index for taking a portion of the referencesignal sequence, N_(RB) ^(DL) is the number of RBs in use, α_(k,1)(p) ismodulation symbols used as a reference symbol for a p-th antenna port ata slot n_(s), k is a subcarrier index used for transmission of areference signal, and N_(RS) ^(interval) is an interval of a start pointbased on a cell ID N_(cell) ^(ID). r_(l,ns)(m) may be a basic sequence,and r_(l,ns)(m′) may be a used sequence.

FIG. 15 shows that a basic sequence in use is cyclic shifted accordingto a cell ID. A basic sequence having a length of M_(max) (i.e., a basicsequence c(i) (i=0, 1, . . . , M_(max)−1)) is generated by the goldsequence generator. Thereafter, a cyclic shift amount N is determinedaccording to the cell ID. Then, the basic sequence is cyclic shifted bythe cyclic shift amount N. In this case, a start point of the usedsequence may always be placed at the same position. From the basicsequence c(i) (i=0, 1, . . . , M_(max)−1), an used sequence cu(i) (i=0,1, . . . , M−1) can be expressed by the following equation:

c _(shift)((i+N·N _(ID) ^(cell))mod(M _(max)−1))=c(i)

cu(i)=c _(shift)(i)  [Equation 4]

where c_(shift)(i) is a sequence obtained by cyclic shifting the basicsequence by the cyclic shift amount N.

Equation 4 above can be expressed in a format of a reference signal forthe 3GPP LTE system in which resources are allocated in an RB unit,which is shown in the following equation.

$\begin{matrix}{\begin{matrix}{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} +}} \\{{{j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)},}}\end{matrix}{{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}}{a_{k,l}^{(p)} = {r_{l,n_{s}}\left( m^{\prime} \right)}}{{m = 0},1,\ldots \mspace{14mu},{{2\; \cdot N_{RB}^{DL}} - 1}}{m^{\prime} = {\begin{pmatrix}{m + {N_{Interval}^{RS}N_{ID}^{cell}} +} \\{N_{RB}^{\max,{DL}} - N_{RB}^{DL}}\end{pmatrix}{{mod}\left( {{2 \cdot N_{RB}^{\max,{DL}}} - 1} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In another embodiment, when a pseudo-random sequence is generated by thegold sequence generator, some sequences generated initially can beexcluded. A sequence having a length of Nc may be removed from aninitially generated gold sequence, and a subsequence sequence may beused as a reference signal sequence. Initial values have a great effecton the initially generated sequences, and thus PAPR propertydeterioration caused by similar initial values can be avoided. This canbe expressed by the following equation.

c′(i)=c(i+Nc)  [Equation 6]

Equation 6 above can be expressed in a form of Equation 1 above, whichis shown in the following equation.

c(i)=(x(i+Nc)+y(i+Nc))mod 2

x(i+31)=(x(i+3)+x(i))mod 2

y(i+31)=(y(i+3)+y(i+2)+y(i+1)+y(i))mod 2  [Equation 7]

A value Nc may be set to a length which is generated sufficientlyrandomly so that a generated pseudo-random sequence is not affected byinitial values. For example, the value Nc may range from 1500 to 1800.

Equation 7 above can be expressed in a format of a reference signal forthe 3GPP LTE system in which resources are allocated in an RB unit byusing the pseudo-random sequence c(i), which is shown in the followingequation.

$\begin{matrix}{\begin{matrix}{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} +}} \\{{{j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)},}}\end{matrix}{{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}}{a_{k,l}^{(p)} = {r_{l,n_{s}}\left( m^{\prime} \right)}}{{m = 0},1,\ldots \mspace{14mu},{{2\; \cdot N_{RB}^{DL}} - 1}}{m^{\prime} = {m + N_{RB}^{\max,{DL}} - N_{RB}^{DL}}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Now, the cross-correlation property between generated pseudo-randomsequences is described.

A pseudo-random sequence G(D) generated using two m-sequences X(D) andY(D) is expressed by the following polynomial form.

G(D)=c ₀ +c ₁ D+c ₂ D ² + . . . G(D)=X(D)⊕Y(D)  [Equation 9]

Herein, a first m-sequence X(D) is I₁(D)/g₁(D), and a second m-sequenceY(D) is I₂(D)/g₂(D). g₁(D) and g₂(D) are primitive polynomials forgenerating X(D) and Y(D), and are defined as follows.

g ₁(D)=1+D ⁻³ +D ⁻³¹

g ₂(D)=1+D ⁻¹ +D ⁻² +D ⁻³ +D ⁻³¹  [Equation 10]

I₁(D) and I₂(D) are initial values for generating X(D) and Y(D), and aredefined by the following equation.

I ₁(D)=1

I ₂(D)=I(CELLID)⊕I(N _(sf))D ⁹  [Equation 11]

Herein, I(CELLID) is an initial value according to a cell ID CELLID, andI(N_(sf))D⁹ is an initial value according to a slot number and an OFDMsymbol number.

In a synchronous environment where timing is identical between multiplecells, neighboring cells have the same slot number and the same OFDMsymbol number. If it is assumed that the same slot number and the sameOFDM symbol number are used, a cross-correlation between pseudo-randomsequences generated in two neighboring cells having different cell IDsCELLID1 and CELLID2 is obtained by the following equation.

$\begin{matrix}\begin{matrix}{{{G_{1}(D)} \oplus {G_{2}(D)}} = {{X(D)} \oplus {Y_{1}(D)} \oplus {X(D)} \oplus {Y_{2}(D)}}} \\{= {{Y_{1}(D)} \oplus {Y_{2}(D)}}} \\{= {{{I_{2,{{cell}\; 1}}(D)}/{g(D)}} \oplus {{I_{2,{{cell}\; 2}}(D)}/{g(D)}}}} \\{= {{\left\lbrack {{I\left( {{CELLID}\; 1} \right)} \oplus {{I\left( N_{sf} \right)}D^{9}}} \right\rbrack/{g(D)}} \oplus}} \\{{\left\lbrack {{I\left( {{CELLID}\; 2} \right)} \oplus {{I\left( N_{sf} \right)}D^{9}}} \right\rbrack/{g(D)}}} \\{= {{{I\left( {{CELLID}\; 1} \right)}/{g(D)}} \oplus}} \\{{{I\left( N_{sf} \right){D^{9}/{g(D)}}} \oplus}} \\{{{{I\left( {{CELLID}\; 2} \right)}/{g(D)}} \oplus}} \\{{I\left( N_{sf} \right){D^{9}/{g(D)}}}} \\{= {{{I\left( {{CELLID}\; 1} \right)}/{g(D)}} \oplus {{I\left( {{CELLID}\; 2} \right)}/{g(D)}}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack\end{matrix}$

The above equation shows that the cross-correlation property isdetermined only by the cell ID. Since there is no change in thecross-correlation property between cells according to changes in theslot number and the OFDM symbol number, it may be difficult to obtain asequence having a good cross-correlation property in this method.

When modulated sequences consisting of modulation symbols obtained byperforming QPSK modulation on generated pseudo-random sequences aredenoted as R1[n] and R2[n] for two cells, respectively, the modulatedsequences can be expressed by the following equation:

R1[n]=S[2n]X1[2n]+jS[2n+1]X1[2n+1]

R2[n]=S[2n]X2[2n]+jS[2n+1]X2[2n+1]  [Equation 13]

where S[n] is a cell common sequence depending on a subframe number andan OFDM symbol number, and X1[n] and X2[n] are cell specific sequencesobtained from each cell ID. A cross-correlation for the above sequencesR1[n] and R2[n] can be obtained by the following equation:

$\begin{matrix}\begin{matrix}{{R\; {1\lbrack n\rbrack}R\; {2\lbrack n\rbrack}^{*}} = \begin{pmatrix}{{{S\left\lbrack {2\; n} \right\rbrack}X\; {1\left\lbrack {2\; n} \right\rbrack}} +} \\{j\; {S\left\lbrack {{2\; n} + 1} \right\rbrack}X\; {1\left\lbrack {{2\; n} + 1} \right\rbrack}}\end{pmatrix}} \\{\begin{pmatrix}{{{S\left\lbrack {2\; n} \right\rbrack}X\; {2\left\lbrack {2\; n} \right\rbrack}} +} \\{j\; {S\left\lbrack {{2\; n} + 1} \right\rbrack}X\; {2\left\lbrack {{2\; n} + 1} \right\rbrack}}\end{pmatrix}^{*}} \\{= {{X\; {1\left\lbrack {2\; n} \right\rbrack}X\; {2\left\lbrack {2\; n} \right\rbrack}^{*}} + {X\; {1\left\lbrack {{2\; n} + 1} \right\rbrack}X\; {2\left\lbrack {{2\; n} + 1} \right\rbrack}^{*}} +}} \\{{j\begin{pmatrix}{{{S\left\lbrack {{2\; n} + 1} \right\rbrack}X\; {1\left\lbrack {{2\; n} + 1} \right\rbrack}{S\left\lbrack {2\; n} \right\rbrack}^{*}X\; {2\left\lbrack {2\; n} \right\rbrack}^{*}} -} \\{{S\left\lbrack {{2\; n} + 1} \right\rbrack}X\; {2\left\lbrack {{2\; n} + 1} \right\rbrack}{S\left\lbrack {2\; n} \right\rbrack}^{*}X\; {1\left\lbrack {2\; n} \right\rbrack}^{*}}\end{pmatrix}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$

where ( ) denotes a complex conjugate. A cross-correlation result of thetwo modulated sequences R1[n] and R2[n] shows that a cell commonsequence component that varies by a subframe number and an OFDM symbolnumber exists without alteration in a Q-axis whereas the cell commonsequence component is removed in an I-axis. Therefore, it is difficultto obtain a good cross-correlation property between cells.

Accordingly, a method is proposed to improve the cross-correlationproperty between generated pseudo-random sequences.

In one embodiment, a start point of a used sequence can be changedaccording to a subframe number and/or an OFDM symbol number. FIG. 16shows that the start point of the used sequence is changed according tothe subframe number and/or the OFDM symbol number. A long pseudo-randomsequence is generated according to each cell ID. A plurality of basicsequences, each having a length of M_(max), capable of supporting amaximum number of RBs are obtained from the long pseudo-random sequenceaccording to the subframe number and the OFDM symbol number. A usedsequence having a length of M, which is used for transmission of anactual reference signal, is obtained from a basic sequence. Accordingly,the cross-correlation property of a reference signal between cells canbe improved.

The reference sequence can be expressed in a format of a referencesignal for the 3GPP LTE system in which resources are allocated in an RBunit, which is shown in the following equation:

$\begin{matrix}{\begin{matrix}{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + l^{\prime}} \right)}}} \right)} +}} \\{{{j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1 + l^{\prime}} \right)}}} \right)},}}\end{matrix}{l^{\prime} = {4\; {N_{RB}^{\max,{DL}} \cdot \left( {{N_{symb}^{DL} \cdot n_{s}} + l} \right)}\mspace{14mu} {and}}}\mspace{11mu} \; {{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 15} \right\rbrack\end{matrix}$

where n_(s) is a slot number within a radio frame, 1 is an OFDM symbolnumber within a slot, r_(l,ns) is a reference signal sequence, N_(RB)^(max,DL) is a maximum number of RBs, m is an index of the referencesignal sequence, and N_(symb) ^(DL) is the number of OFDM symbolsincluded in a slot. A basic sequence c(i) generated by the gold sequencegenerator is initialized with N_(ID) ^(cell)+1 at the start of each OFDMsymbol.

In another embodiment, initial values used in generation of a basicsequence can be changed to improve the cross-correlation property. If asubframe number and/or an OFDM symbol number are identical in asynchronous environment where timing is identical between multiplecells, the initial values depending on the subframe number and/or theOFDM symbol number are similar between cells, which may result in a poorcorrelation property. In addition, in an asynchronous environment, atransmit time difference between neighboring cells needs to be takeninto consideration so that the initial values are not contiguouslyidentical.

The initial values may vary differently according to changes in thesubframe number and/or the OFDM symbol number between cells. Forexample, a first cell may be configured so that an initial value isincreased or decreased as the number of OFDM symbol number is increased,and a second cell may also be configured so that an initial value isincreased or decreased as the OFDM symbol number is increased. Forexample, a cell having a cell ID of CELLID1 is configured so that aninitial value is increased by n as the OFDM symbol number is increasedby 1. In addition, a cell having a cell ID of CELLID2 is configured sothat an initial value is increased by n+1 as the OFDM symbol number isincreased by 1.

The OFDM symbol number may be extended in a radio frame unit instead ofexisting within a subframe or a slot, so that changes of initializationare different as the OFDM symbol number varies. If N_(sym) OFDM symbolsexist for each subframe, a q-th OFDM symbol number of a k-th subframe ofa radio frame can be expressed by k*N_(sym)+q.

In a system in which the number of OFDM symbols included in eachsubframe varies, a maximum number N_(sym,max) of OFDM symbols for eachsubframe can be defined. In this case, the q^(th) OFDM symbol number ofthe k^(th) subframe of the radio frame can be expressed byk*N_(sym,max)+q. The reason above is to allow each OFDM symbol to have aunique OFDM symbol number in one radio frame.

The gold sequence generator may increase or decrease an initial value ofan m-sequence by a predetermined interval as the OFDM symbol number isincreased. For example, a cell having a cell ID of CELLID1 is allowed toincrease an initial value by a predetermined value such as CELLID1 orCELLID1+1 as the OFDM symbol number is increased by 1. In addition, acell having a cell ID of CELLID2 is allowed to increase an initial valueby a predetermined value such as CELLID2 or CELLID2+1 as the OFDM symbolnumber is increased by 1. However, this may be problematic when a cellID between cells has a difference of about two times. For example, ifCELLID1=5, CELLID2=11 and the predetermined values are CELLID1+1 andCELLID2+1 respectively, then initial values increased as the OFDM symbolnumber is increased are respectively 6 and 12, which shows a differenceof two times. This can be expressed in a binary format in which one bitis shifted. This is because 6 is ‘0110’ in a binary format, and 12 is‘1100’ in a binary format. When one bit is shifted, thecross-correlation property deteriorates in case of using QPSK modulationdue to overlapping between an I-axis component of a reference signal ofa first cell and a Q-axis component of a reference signal of a secondcell.

Therefore, as the OFDM symbol number and/or the subframe number areincreased, there is a need to set the initial values such that anincrement of one cell is not two times an increment of another cell.This can be easily implemented by allowing the initial value to beincreased or decreased in odd multiples as the OFDM symbol number and/orthe subframe number are increased. For example, an initial value of thegold sequence generator having a cell ID of n is allowed to be increasedor decreased by (2n+1) times as the OFDM symbol number is increased ordecreased.

This can be expressed in a format of a reference signal for the 3GPP LTEsystem in which resources are allocated in an RB unit, which is shown inthe following equation.

$\begin{matrix}{\begin{matrix}{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} +}} \\{{{j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)},}}\end{matrix}{{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 16} \right\rbrack\end{matrix}$

Herein, n_(s) is a slot number within a radio frame, l is an OFDM symbolnumber within a slot, r_(l,ns) is a reference signal sequence, andN_(RB) ^(max,DL) is a maximum number of RBs. In this case, the sequencegenerator can be initialized by the following equation:

c _(init)=2⁹·(l′1)·(2·N _(ID) ^(cell)+1)+N _(ID) ^(cell)  [Equation 17]

where l′ is defined as 8n_(s)+l and is an OFDM symbol number within aradio frame.

Meanwhile, a cross-correlation between pseudo-random sequences dependson a binary addition result of initial values used to generate twopseudo-random sequences, which is shown in the following equation.

$\begin{matrix}\begin{matrix}{{{G_{1}(D)} \oplus {G_{2}(D)}} = {{X(D)} \oplus {Y_{1}(D)} \oplus {X(D)} \oplus {Y_{2}(D)}}} \\{= {{Y_{1}(D)} \oplus {Y_{2}(D)}}} \\{= {{{I_{2,{{cell}\; 1}}(D)}/{g(D)}} \oplus {{I_{2,{{cell}\; 2}}(D)}/{g(D)}}}} \\{= {\left( {{I_{2,{{cell}\; 1}}(D)} \oplus {I_{2,{{cell}\; 2}}(D)}} \right)/{g(D)}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 18} \right\rbrack\end{matrix}$

Therefore, if the pseudo-random sequences are generated by varying theinitial values according to each OFDM symbol number, a goodcross-correlation property is obtained when the binary addition resultof the initial values of the respective cells varies as the OFDM symbolnumber varies. This implies that an initial value c_(init)(n₁, l) of afirst cell and an initial value c_(init)(n₂, l) of a second cell vary asthe OFDM symbol number l varies. Herein, n₁ is a cell ID of the firstcell, and n₂ is a cell ID of the second cell. In addition, inconsideration of QPSK modulation, the good cross-correlation propertycan be obtained when (2·c_(init)(n₁,l)⊕c_(init)(n₂, l) andc_(init)(n₁,l)⊕(2·c_(init)(n₂,l)) vary according to the OFDM symbolnumber l.

FIG. 17 shows setting of initial values of a gold sequence generator. 31bits of initial values of the second LFSR are divided into two regions(i.e., a region #1 and a region #2). Each region consists of 14 bits.The region #2 is positioned in an LSB side. Any value can be set to theremaining 4 bits from an MSB. Each of the region #1 and the region #2includes a binary sequence of a cell ID. In the region #1, the binarysequence of the cell ID is cyclic shifted by a first cyclic shift m₁according to the OFDM symbol number l. In the region #2, the binarysequence of the cell ID is cyclic shifted by a second cyclic shift m₂according to the OFDM symbol number l. For example, in the region #1,the binary sequence of the cell ID can be cyclic shifted by a cyclicshift lm₁, and in the region #2, the binary sequence of the cell ID canbe cyclic shifted by a cyclic shift lm₂. By dividing the initial valuesinto two regions and by including a binary sequence of a cell ID forwhich a different cyclic shift is used for in each region,c_(init)(n₁,l)⊕c_(init)(n₂,l) is allowed to be changed according to theOFDM symbol number l.

If b₁ denotes a size of the region #1 and b₂ denotes a size of theregion #2, then b₁=b₂=14. The sizes of the regions #1 and #2 can bearbitrarily defined within a range of the initial value. To increase ageneration period of a gold sequence, b₁ and b₂ may be set to berelatively prime. In addition, m₁ and b₁ as well as m₂ and b₂ may alsobe set to be relatively prime.

This can be expressed in a format of a reference signal for the 3GPP LTEsystem in which resources are allocated in an RB unit, which is shown inthe following equation:

$\begin{matrix}{\begin{matrix}{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} +}} \\{{{j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)},}}\end{matrix}{{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 19} \right\rbrack\end{matrix}$

where c_(init)=2¹⁴CS₁₃(2N_(ID) ^(cell)+1, 11·l′)+CS₁₄(2N_(IC) ^(cell)+1,3·l′) at the start of each OFDM symbol, l′=2·n_(s)+└(2·l)/N_(symb)^(DL)┘, and CS_(b)(M,a)=(2^(a mod b)·M)mod2^(b)+└(2^(a mod b)·M)/2^(b)┘.l′ is an OFDM symbol number within a radio frame, CS_(b)(M, a) is acyclic shift function, and └x┘ denotes a floor function which give alargest integer smaller than x.

Although it has been described that the proposed sequence is used for adownlink reference signal of the 3GPP LTE/LTE-A, the proposed sequencecan also be used for an uplink reference signal. In addition, althoughPAPR and cross-correlation properties have been described for areference signal between cells, these properties can also be equallyused for a reference signal between UEs and/or between antennas.

A reference signal used for the proposed sequence may be either a cellcommon reference signal or a UE specific reference signal.

FIG. 18 is a flowchart showing a method of transmitting a referencesignal according to an embodiment of the present invention. This methodmay be performed by a transmitter. The transmitter may a part of a BSwhen a downlink reference signal is transmitted, or may a part of a UEwhen an uplink reference signal is transmitted. In step S510, areference signal sequence is generated. The reference signal sequencecan be defined by the following equation.

$\begin{matrix}{\begin{matrix}{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} +}} \\{{{j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)},}}\end{matrix}{{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 20} \right\rbrack\end{matrix}$

Herein, n_(s) is a slot number within a radio frame, 1 is an OFDM symbolnumber within a slot, r_(l,ns) is a reference signal sequence, andN_(RB) ^(max,DL) is a maximum number of RBs. The pseudo-random sequencec(i) can be defined by Equation 7 above. Herein, the m-sequence x(i) canbe initialized with initial values expressed by x(0)=1, x(i)=0, i=1, 2,. . . , 30, and the m-sequence (y) can be initialized with initialvalues obtained from (2N_(ID) ^(cell)+1), where N_(ID) ^(cell) is a cellID. The initial values of the m-sequence y(i) varies as the OFDM symbolnumber l varies. Therefore, the initial values of the m-sequence y(i)can be obtained from l(2N_(ID) ^(cell)+1).

In step S520, a portion or entirety of the reference signal sequence ismapped to at least one RB. One RB can include 12 subcarriers. In case ofa cell common reference signal, two modulation symbols of the referencesequence can be mapped to two subcarriers within one RB. In case of a UEspecific reference signal, three modulation symbols of the referencesignal sequence can be mapped to three subcarriers within one RB.

In step S530, the reference signal is transmitted using the RB. Aproposed reference signal sequence provides improved PAPR andcross-correlation properties. Therefore, transmit power efficiency of atransmitter can be increased, and a receiver can be provided with higherdetection performance.

FIG. 19 is a block diagram showing a transmitter and a receiverimplementing for a method of transmitting and receiving a referencesignal. A transmitter 800 includes a data processor 810, a referencesignal generator 820, and a transmit circuitry 830. The data processor810 processes an information bit to generate a transmit signal. Thereference signal generator 820 generates a reference signal. Thereference signal generation of FIG. 18 may be performed by the referencesignal generator 820. The transmit circuitry 830 transmits the transmitsignal and/or the reference signal.

A receiver 910 includes a data processor 910, a channel estimator 920,and a receive circuitry 930. The receive circuitry 930 receives areference signal and a receive signal. The channel estimator 920estimates a channel by using the received reference signal. The dataprocessor 910 processes the receive signal by using the estimatedchannel.

Although a proposed sequence is used for a reference signal as anexample in the aforementioned embodiment, the proposed sequence can beused for various signals. For example, the proposed sequence can be usedfor a scrambling code, a synchronous signal, a preamble, a masking code,etc. Based on the pseudo-random sequence c(i) of Equation 7, a basesequence of Equation 20 may be generated. The m-sequence y(i) for thepseudo-random sequence c(i) may be initialized with initial valuesobtained from (2N_(ID) ^(cell)+1) where N_(ID) ^(cell) is a cell ID. Thebase sequence may be applied with a target signal or a target code. Toapply the base sequence with the target signal or the target code, aportion or entirety of the reference signal sequence may be usedaccording to allocated resources or the length (or size) of the targetsignal or the target code. Applied sequence is transmitted. Thetransmitted sequence may be used as various applications by a receiver.

The present invention can be implemented with hardware, software, orcombination thereof. In hardware implementation, the present inventioncan be implemented with one of an application specific integratedcircuit (ASIC), a digital signal processor (DSP), a programmable logicdevice (PLD), a field programmable gate array (FPGA), a processor, acontroller, a microprocessor, other electronic units, and combinationthereof, which are designed to perform the aforementioned functions. Insoftware implementation, the present invention can be implemented with amodule for performing the aforementioned functions. Software is storablein a memory unit and executed by the processor. Various means widelyknown to those skilled in the art can be used as the memory unit or theprocessor.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

1. A method of transmitting a sequence for a reference signal in awireless communication system, the method comprising: initializing apseudo-random sequence c(i) with an initial value, where i=0, 1, . . . ,M_(max)−1, and K_(max) is a length of the pseudo-random sequence c(i);generating a sequence which is defined by${{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)}}},{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}$where n_(s) is a slot number within a radio frame, l is an orthogonalfrequency division multiplexing (OFDM) symbol number within a slot,N_(RB) ^(max,DL) is a maximum number of resource blocks (RBs), and c(2m)and c(2m+1) are defined by the pseudo-random sequence c(i); mapping aportion or entirety of the sequence to at least one RB; and transmittingthe mapped sequence in the at least one RB, wherein the initial value isobtained on the basis of the slot number and (2N_(ID) ^(cell)+1), whereN_(IC) ^(cell) is a cell identifier.
 2. The method of claim 1, whereinthe pseudo-random sequence c(i) is defined byc(i)=(x(i+Nc)+y(i+Nc))mod 2x(i+31)=(x(i+3)+x(i))mod 2y(i+31)=(y(i+3)+y(i+2)+y(i+1)+y(i))mod 2 where x(i) and y(i) arem-sequences and Nc is a constant.
 3. The method of claim 2, wherein them-sequence x(i) is initialized with x(i)=1 at i=0, x(i)=0 at i=1, 2, . .. , 30, and the m-sequence y(i) is initialized with the initial value.4. The method of claim 3, wherein the Nc is a value in range from 1500to
 1800. 5. The method of claim 1, wherein the reference signal is acell common reference signal or a user equipment (UE) specific referencesignal.
 6. A transmitter comprising: a reference signal generatorconfigured to generate a sequence for a reference signal; and a transmitcircuitry configured to transmit the sequence, wherein the referencesignal generator is configured to: initialize a pseudo-random sequencec(i) with an initial value, where i=0, 1, . . . , M_(max)−1, and M_(max)is a length of the pseudo-random sequence c(i), and generate thesequence which is defined by${{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)}}},{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}$where n_(s) is a slot number within a radio frame, 1 is an orthogonalfrequency division multiplexing (OFDM) symbol number within a slot,N_(RB) ^(max,DL) is a maximum number of resource blocks (RBs), and c(2m)and c(2m+1) are defined by the pseudo-random sequence c(i), wherein theinitial value is obtained on the basis of the slot number and (2N_(ID)^(cell)+1), where N_(ID) ^(cell) is a cell identifier.
 7. Thetransmitter of claim 6, wherein the pseudo-random sequence c(i) isdefined byc(i)=(x(i+Nc)+y(i+Nc))mod 2x(i+31)=(x(i+3)+x(i))mod 2y(i+31)=(y(i+3)+y(i+2)+y(i+1)+y(i))mod 2 where x(i) and y(i) arem-sequences and Nc is a constant.
 8. The transmitter of claim 7, whereinthe m-sequence x(i) is initialized with x(i)=1 at i=0, x(i)=0 at i=1, 2,. . . , 30, and the m-sequence y(i) is initialized with the initialvalue.
 9. A receiver comprising: a receive circuitry configured toreceive a reference signal and a receive signal; a channel estimatorconfigured to estimate a channel by using the reference signal; and adata processor configured to process the receive signal by using thechannel, wherein a sequence for the reference signal is defined by${{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)}}},{m = 0},1,\ldots \mspace{14mu},{{2\; N_{RB}^{\max,{DL}}} - 1}$where n_(s) is a slot number within a radio frame, 1 is an orthogonalfrequency division multiplexing (OFDM) symbol number within a slot,N_(RB) ^(max,DL) is a maximum number of resource blocks (RBs), and c(2m)and c(2m+1) are defined by a pseudo-random sequence c(i), where i=0, 1,. . . , M_(max)−1, and M_(max) is a length of the pseudo-random sequencec(i), wherein the pseudo-random sequence c(i) is generated by a goldsequence generator which is initialized with initial values obtained onthe basis of the slot number and (2 N_(ID) ^(cell)+1), where N_(ID)^(cell) is a cell identifier.
 10. The receiver of claim 9, wherein thepseudo-random sequence c(i) is defined byc(i)=(x(i+Nc)+y(i+Nc))mod 2x(i+31)=(x(i+3)+x(i))mod 2y(i+31)=(y(i+3)+y(i+2)+y(i+1)+y(i))mod 2 where x(i) and y(i) arem-sequences and Nc is a constant.
 11. The receiver of claim 10, whereinthe m-sequence x(i) is initialized with x(i)=1 at i=0, x(i)=0 at i=1, 2,. . . , 30, and the m-sequence y(i) is initialized with the initialvalue.